Method of heat-treating tungstennickel alloys



C. VERDI Oct. 13, 1953 METHOD OF HEAT-TREATING TUNGSTEN-NICKEL. ALLOYSFiled July 26 1951 2 Sheets-Sheet l INTERIM TEMPBQATURE 4002: 50cc aIan: M:

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u O m 1 m 7 0 u A mmmm: M22 mwmumw lf uu w x Q A E 6 N w Patented Oct.13, 1953 METHOD OF HEAT-TREATING TUNGSTEN- NICKEL ALLOYS Columbus Verdi,Waukegan, Ill., asslgnor to Fansteel Metallurgical Corporation, NorthChicago, III., a corporation of New York Application July 26, 1951,Serial No. 238,598

Claims.

1 This invention relates to high density tungsten alloys and morespecifically to a method of improving the physical and mechanicalproperties oi certain high density alloys of tungsten and nickel by aheat treatment of the alloys.

. nickel containing from about 5% to about 45% nickel, when subjected tothe heat treatment described hereinafter, exhibit an unexpectedsubstantial increase in the mechanical properties of the alloys.

The alloys or compositions are produced by conventional powdermetallurgy methods. A thorough and intimate mixture of the tungsten andnickel powder is prepared by any desired method. For example, the metalpowders may be intimately mixed by a prolonged ball milling. or, inthose instances where the nickel content is low. the nickel may be addedto the tungsten powder in the form of a reducible nickel compound suchas nickel chloride which is subsequently reduced in hydrogen to formnickel, or the mixture may be prepared by forming an intimate mixture oftungstic acid and a reducible nickel salt such as nickel chloride andreducing the tungstic acid and nickel salt simultaneously in anatmosphere 01' hydrogen. The metal powder is then pressed into thedesired shape and sintered in a hydrogen atmosphere or in a dissociatedammonia gas atmosphere, in accordance with conventional practices.Sintering temperatures for these tungsten-nickel alloys lie within therange of from about 1250 C. to about 1450" C. The period of sinteringwill vary inversely with the specific sintering temperature employed.

In general, it has been discovered that sintering temperatures in therange from about 1300 C. to about 1400 C. produce alloys having maximumhardness and mechanical characteristics in the as sintered condition.Higher sintering temperatures promote a higher degree of grain growthand the resulting products exhibit, in most instances, a slightly lowerhardness and mechanical strength.

I have discovered that these alloys when heated to a temperature aboveabout 1000 C., preferably to about 1200 C., and when rapidly cooled,

as by quenching in a liquid or in a gas stream. from this temperatureexhibit an appreciable increase in the mechanical strength without anappreciable effect on the hardness. In general, the transverse ruptureincreases about 20% to 25% while the averagedecrease in hardness is fromabout 1 to 2 points on the Rockwell C scale.

I have also discovered that the mechanical strength of the alloys may befurther appreciably increased by subsequently subjecting the alloys to aheat treatment consisting of heating to a temperature between about 500C. and 700 C. for from about 1 hour to about 2 hours. This treatmentresults in a further increase of from 20% to 25% in the transverserupture strength and also increases the hardness to about that of thealloys before heat treatment.

I have further discovered that the hardness may be increased bysubjecting the alloys to a heat treatment which consists of heating thealloys to a temperature between about 600 C. and 900 C. for from about 1hour to 2 hours. Where the temperature does not exceed about 800 C., theincrease in hardness is obtained without decreasing the strength belowthat of the sintered alloy. Where the strength is not controlling and aharder material is desired, the heat treatment maybe extended to about900 C.

In order to illustrate the improvements in the physical and mechanicalcharacteristics of the tungsten-nickel alloys when subjected to the heattreatment of this invention a series of such alloys were prepared. Thenickel content of this series of alloys varied from 5.18% nickel to43.60% nickel. Bars were prepared from the various powder mixtures bypressing the powder at about tons, per square inch. Groups of pressedbars of the various powder mixtures were subsequently sintered attemperatures between 1300" C. and 1325 0., between 1400 C. and 1425 C..and between 1440 C. and l450 C. Sintering was carried out in hydrogenand in dissociated ammonia gas. The bars were approximately 5" in lengthand varied from about 0.2" to about 0.3" in width and thickness.

The hardness of the bars in the as sintered condition was determined ona standard hardness testing apparatus employing the Rockwell 0 scale.The series of bars of the various compositions were mounted andsubjected to predetermined loading so as to determine the transverserupture strength. Another series of the bars were heated in hydrogen toa temperature of about 1200 C. and maintained at this tem-' perature forabout 15 minutes to insure uniform heating throughout the bars. Thisseries of bars such temperature for about one hour and finally Vremoving the bars from the furnace and allowing them to cool in theatmosphere. Groups of bars were subjected to such heat treatment attemperatures of 100 C. to 900 C. in intervals of 100 C. Each group ofbars after being sub- Jected to its heat treatment was tested forhardness and transverse rupture.

The data so obtained was employed in plotting the graphs of-Figs. 1 and2.

In Fig. 1 the transverse rupture strength is plotted against the heattreatment. It will be noted that the quenching treatment increases thetransverse rupture strength an average of from 25% to 30% for eachseries of sintering temperatures.

In Fig. 2 the hardness is plotted against the heat treatment. It will benoted that quenching for all practical purposes, effects no alterationof the hardness.

Treatment of the alloys by heating them to temperatures of from about100 C. to about 500 C. generally decreases the transverse rupturestrength to a value which lies between the strength of the alloys intheir as sintered condition and in their quenched condition. Similarly.in most instances, the hardness of the alloys subjected to thesubsequent heat treatment within this temperature range varies slightlyand lies between the hardness of the alloys in their as crease in thehardness of the alloys. Heat treatment in the range or from 800 C. to900 C. etiects a further increase in the hardness accompanied by adecrease in the mechanical strength.

The explanation for the unusual increase in strength without anaccompanying increase in hardness eflfected by the quenching and heattreatment is not apparent. The phase diagram of these tungsten-nickelalloys does not indicate that the alloys rich in tungsten areprecipitation hardenable.

Iclaim:

1. The method or increasing substantially the mechanical strengthwithout increasing appreciably the hardness of sintered tungsten-nickelalloys containing from about 5% to about 45% nickel and the balancetungsten, which includes heating the sintered alloy to a temperaturebetween about 1000 C. and 1200 C., quenching the heated alloy andsubjecting the quenched alloy to heat treatment at a temperature of fromabout 500 C. to about 700 C.

2. The method of increasing substantially the mechanical strengthwithoutincreasing appreciably the hardness of sintered tungsten-nickel alloyscontaining from about 5% to about nickel and the balance tungsten, whichincludes heating the sintered alloy to a temperature between about 1000C. and 1200 C., quenching the heated alloy and subjecting the quenchedalloy to heat treatment at a temperature of the order of 600 C.

3. An an article of manufacture, a sintered tungsten-nickel alloyconsisting essentially of from about 5% to about 45% nickel and thebalance tungsten, formed by heating the sintered alloy to a temperaturebetween about 1000 C.

I and 1200 C., quenching the alloy from such temsintered condition andin their quenched condition.

Higher heat treatment effects a further increase in the strength ascompared to the strength in the quenched condition and the increasevaries from about 20% to about 40% depending upon the sinteringtemperature.

From the graphs it is apparent that the heat treatment at a temperatureof about 600 C. produces the optimum increase in the strength. Heattreatment at this temperature efiects an increase in strength ascompared to the strength in the sintered condition of from about toabout 80% depending upon the sintering temperature.

In general, the hardness of the alloys begins to increase with a heattreating temperature of about 500 C. and the hardness of the alloy whensubjected to heat treatment at 600 C. is the same as the hardness of thealloys in their as sintered condition. It is to be understood that thisvariation in hardness is within a range of about 2 points on theRockwell C scale.- As the temperature is increased above 600 C., thtransverse rupture strength rapidly decreases and the hardness rapidlyincreases. ,The optimum heat treating temperature range for theproduction of maximum transverse rupture strength without an appreciablealteration of the hardness lies in the temperature range of from about500 C. to about 700 C. In most instances heat treatment at 800 C.results in a strength about that of the alloys in their as sinteredcondition; however, such a temperature produces a substantial inperatureand heat treating the quenched alloy at a temperature of from about 500C. to about 700 C. and being characterized by having a substantiallygreater mechanical strength than, but

by having approximately the same hardness as, a corresponding sinteredalloy in its as sintered condition.

4. As an article of manufacture, a sintered tungsten-nickel alloyconsisting essentially of from about 5% to about 45% nickel and thebalance tungsten, formed by heating the sintered alloy to a temperaturebetween about1000 C. and 1200 C., quenching the alloy from suchtemperature and heat treating the quenched alloy at a temperature of theorder of 600 C. and being characterized by having a substantiallygreater mechanical strength than, but by having approximately the samehardness as, a corresponding sintered alloy in its as sinteredcondition. 5. The method of increasing substantially the mechanicalstrength without increasing appreciably the hardness of a sinteredtungsten-nickel alloy containing from about 5% tov about 45% nickel andthe balance tungsten, which has been sintered at a temperature betweenabout 1300 C. to 1325 C., said method comprising heating the sinteredalloy-to a temperature between about 1000 C. and 1200 C., quenching theheated alloy and subjecting the quenched alloy'to heat treatment at atemperature of the order of 600 C.

COLUMBUS VERDI.

' References Cited in the file of this patent I Transactions of TheAmerican Societyior Metals, volume 28, pages 619-643, especially p es636-642 (1940). I

1. THE METHOD OF INCREASING SUBSTANTIALLY THE MECHANICAL STRENGTHWITHOUT INCREASING APPRECIABLY THE HARDNESS OF SINTERED TUNGSTEN-NICKELALLOYS CONTAINING FROM ABOUT 5% TO ABOUT 45% NICKEL AND THE BALANCETUNGSTEN, WHICH INCLUDES HEATING THE SINTERED ALLOY TO A TEMPERATUREBETWEEN ABOUT 1000* C. AND 1200* C., QUENCHING THE HEATED ALLOY ANDSUBJECTING THE QUENCHING ALLOY TO HEAT TREATMENT AT A TEMPERATURE OFFROM ABOUT 500* C. TO ABOUT 700* C.